I just used the /charge in N minutes Slack endpoint for the first time. It looks like it will be perfect. If I got to pick, I would prefer something in the Beeminder UI, but the advantage of the Slack functionality is this it’s already done. 
Thanks folks!
Insti, there’s some examples over at http://forum.beeminder.com/t/intermittent-frog-task-difficulty/4070.